The best way is to use a VM (Virtual Machine) such as VirtualBox or VMware. This will allow you to run a completely self-contained installation of Windows in which you can safely run untrusted software. Once the offer approves and you've paid things forward a bit, you can simply revert the VM to your snapshot, knowing that everything is now exactly as it was!

The alternative is to use a sandbox environment such as Sandboxie, which is theoretically a little bit less secure, but doesn't require as much hard drive space. Some people find sandbox environments confusing and some installation offers may not work properly in them. If you don't feel comfortable doing these sorts of things you can always skip/ignore the download/install offers, but it's really not difficult. It's a great way to earn some extra cash or points!